Ceramics have been attracting notice as a heat-resistant structural materials replacing metals, since they have heat-resistant and oxidation-resistant properties superior to those of metal materials, and are also excellent in heat-insulating property. However, since ceramics are constituted by covalent bond or ionic bond, and can not be deformed or expanded by dislocations as metal materials, there occur stress concentrations at minute defects within a material and flaws on a surface. Hence ceramics are easily broken and have disadvantages that they are very fragile and inferior in fracture toughness.
Mullite (3Al.sub.2 O.sub.3. 2SiO.sub.2) which has the heat-resistant property and is excellent in thermal-shock resistance shows a uniform thermal expansion and small variations in mechanical properties due to temperature, and has a strength nearly identical to that of silicon carbide at 1350.degree. C., but it is inferior in the fracture toughness like other ceramics.
The resistance of a material against brittle fracture is generally indicated by the value of fracture toughness K.sub.Ic. The K.sub.Ic 's of conventional mullite and silicon nitride materials are 1.5-1.8 MN/m.sup.3/2 and 5-7 MN/m.sup.3/2, respectively, and these values are extremely low even compared with 34 MN/m.sup.3/2 for aluminum alloys which are considered to be relatively fragile among metal materials. In order to apply ceramics for reciprocating engines or gas-turbine engines as engineering ceramics, it is necessary to increase the value of fracture toughness, and especially it is preferred to make that value not less than 10 MN/m.sup.3/2.
Accordingly, in order to improve fragility of the structural ceramics, various techniques have been investigated. Among them, particle-dispersion reinforcing method in which various particles are mixed and dispersed within a ceramic matrix and fiber reinforcing method in which various kinds of fibers are dispersed within a ceramic matrix have been attracting notice.
Fibers for fiber-reinforced ceramics (termed hereinafter FRC) are roughly divided into a short-fiber type and a long-fiber type. As long fibers, there are glass fibers, metal fibers, carbon fibers, ceramic fibers and the like. Carbon fibers are suitable for a composite since they have high strength and high modulus of elasticity, but they have a disadvantage that they are not resistant against oxidation. Ceramic fibers, such as silicon carbide, alumina and the like, which are made by spinning organic raw materials and being subjected to heat treatment, have high melting points and are most frequently being used. Short fibers indicate whiskers which are needle-like single crystals or fibers chopped from a long fiber. Whiskers show ideal strength as fibers for FRC, but they have disadvantages that it is difficult to uniformly disperse them within a matrix and they are expensive.
As to ceramics for a matrix, many ceramics ranging from oxides to non-oxides, such as Al.sub.2 O.sub.3, mullite, ZrO.sub.2, Si.sub.3 N.sub.4, SiC, glass and the like, have been tried to make composites with fibers.
As to patent references about fiber-reinforced ceramic materials, there are those about a sintered body in which silicon carbide short fibers are mixed with spinel (MgO.Al.sub.2 O.sub.3) (JP Patent Kokai Publication No. 62-119175 (1987)), a sintered body in which silicon carbide short fibers are mixed with alumina (JP Patent Kokai Publication No. 62-119174 (1987)), an SiC composite reinforced by carbon continuous fibers (JP Patent Kokai Publication No. 61-247663 (1986)), a ceramic composite material in which carbon fibers are added to a metal oxide or a metal carbide and sintered simultaneously with pressurizing (JP Patent Kokai Publication No. 50-136306 (1975)), a ceramic composite material reinforced by silicon carbide fibers (JP Patent Kokoku Publication No. 62-35996 (1987)) and the like.
The mechanism for the increase of fracture toughness in ceramics by particle dispersion is considered such that an amount energy for further advancing a front end of a crack is dispersed or absorbed by particles for reinforcement in a certain manner, and a stress relaxation phenomenon occurs. As an example of the fracture-toughness relaxation, there is an example in which TiC particles are dispersed in Si.sub.3 N.sub.4.